Power consuming clients that link to a utility power grid and draw power therefrom have to limit the amount of disturbance they cause at the grid coupling point so that other customers linked to the grid can rely on a power at the coupling point that has at least certain characteristics (e.g., a limited amount of harmonics, a limited amount of unbalance, etc.). To this end a series of regulations (e.g., IEEE 519) have been adopted that specify grid linkage/power usage requirements.
AC power delivered to coupling points via grid lines (i.e., supply lines) is usually not in a condition that can be used by end users and therefore the power at the coupling point must be converted so as to have characteristics required by the end users. For instance, grid AC power is often converted from AC to DC via a rectifier and then back to AC by an inverter where the amplitude and frequency is altered by the AC-DC-AC conversion and the resulting power is in a form useable to power end user equipment (e.g., motors, computers, office equipment, etc.).
To convert supply line AC power to DC power, the power conversion industry has developed various converter topologies and methods. For instance, one common converter topology includes a six-pulse full wave converter. Six pulse converters are advantageous because they are relatively simple and inexpensive to construct. Unfortunately, six-pulse converters have been known to generate high levels of harmonics (e.g., fifth, seventh, third, eleventh, thirteenth, etc.) on linked supply lines which render these converters unusable under certain circumstances or useable only if other conditioning hardware is used therewith. Here, the other conditioning hardware adds expense to the overall system.
Other converter types that overcome some of the shortcomings of the 6 pulse type include a 12 pulse converter and an 18 pulse converter. As well known in the power conversion industry, 12 and 18 pulse converters are able to reduce harmonic distortion when controlled in certain ways and when used to convert balanced supply line voltages. Unfortunately, when supply line voltages are unbalanced, it has been observed that 12 and 18 pulse conversion can result in significant harmonic distortion.
Still one other converter type is generally referred to as an active converter where converter switching devices are actively controlled to facilitate four quadrant operation (i.e., where the converter can be used in a bi-directional manner—as a converter from the grid to the DC bus or as an inverter from the bus to the grid). In addition to other advantages, active converters can reduce supply line harmonics when linked with balanced supply lines such that IEEE 519 standards are met. Unfortunately, it has been observed that when supply line voltages are unbalanced, active conversion can generate second harmonics that exceed tolerable levels.
In addition, when the supply line voltages are unbalanced, active conversion often results in unbalanced current draw. Converter components are usually rated for use with specific maximum or steady state currents and therefore, where currents drawn are unbalanced, the phase of the conversion hardware carrying the highest current must be used to limit conversion rate. In other words, once the current through one phase reaches the rated current level, the converter capacity must be limited to protect that phase despite the fact that the other two phases may have current levels far below the rated level.
Moreover, it has been observed that under certain circumstances unbalanced supply line voltage causes increased voltage ripple on the DC bus (i.e., the link between the converter and the inverter in an AC-DC-AC conversion topology) which can cause increased heating and can shorten the useful life of conversion hardware components as well s the useful life of inverter components linked to the DC bus.
Thus, it would be advantageous to have an AC-DC conversion configuration that could simply and inexpensively maintain supply line harmonics including the second harmonic to below tolerable threshold levels and that could minimize DC bus voltage ripple even where supply line voltages are unbalanced.